Could life be hiding in the oceans under the ice of Europa and Enceladus?

Possibly, and Saturn's moon Enceladus has given the most concrete hints. In 2005 the Cassini spacecraft discovered water-rich plumes venting from its south polar region, jets of water vapour, molecular hydrogen, salt crystals and ice particles that total about 200 kilograms every second. The moon's wobble as it orbits Saturn suggests its whole icy crust is detached from the rocky core, which means a global ocean lies beneath the surface.

Then Cassini did something bold. In 2015 it flew within 48.3 km of the south pole and straight through a plume. Its mass spectrometer detected molecular hydrogen, most likely the result of hydrothermal activity beneath the surface. That matters because hydrogen is a fuel. Microbes, if any lived there, could combine it with dissolved carbon dioxide to get energy, a reaction called methanogenesis that sits at the root of the tree of life on Earth. In 2023, astronomers reported phosphates too, completing the set of basic chemical ingredients for life: carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus.

Jupiter's moon Europa tells a similar story with less direct sampling. The scientific consensus is that liquid water exists beneath its surface, kept liquid by tidal flexing. These are signs of habitable conditions, and the Europa ocean is described as one that could conceivably harbor extraterrestrial life: whether anything actually lives in either ocean is the open question.